1. Field
This disclosure is generally related to manufacturing a solar cell. More specifically, this disclosure is related to integrating a silicon-based dielectric stack for surface and bulk passivation with screen-printing technology for a Si-epitaxial thin-film solar cell manufacturing application.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal bandgaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction, generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
FIG. 1 presents a diagram illustrating an exemplary homojunction solar cell based on a crystalline-Si substrate (prior art). Solar cell 100 includes front electrodes 102, an n+ crystalline-Si emitter layer 104, a p-type doped crystalline-Si substrate 106, and an Al backside electrode 108. Arrows in FIG. 1 indicate incident sunlight. For homojunction solar cells, minority-carrier recombination at the cell surface due to the existence of defect states and dangling bonds can significantly reduce the solar cell efficiency; thus, a good surface passivation process is needed. For conventional homojunction solar cells which use crystalline-Si as an active layer, hydrogen-rich silicon-nitride (SiNx:H) has been widely used as a passivating material. Note that in this type of solar cell, minority carrier recombination inside the bulk Si is the dominant effect, and SiNx:H can effectively passivate the bulk Si by hydrogenation of defects in the bulk Si. Also, the use of SiNx:H can be an adequate technique to passivate the surface of the emitter layer by field-effect passivation.
However, the solar cell demonstrated in FIG. 1 is based on a crystalline-Si substrate whose thickness can be between 200 μm and 300 μm. Due to the soaring cost of Si material, the existence of such a thick crystalline-Si substrate significantly increases the manufacture cost of solar cells; therefore, solar cells based on thin-film technology have been gaining popularity. For a Si thin-film solar cell, the thickness of the Si base film can be between 20 μm and 80 μm, which are typically less than the minority-carrier diffusion length at normal carrier lifetimes. As a result, surface recombination becomes a dominant effect for solar cell efficiency. To achieve high efficiency, the surface recombination velocity (SRV), which is a measure of the minority-carrier surface recombination rate, needs to be less than 1×103 cm/second, and SiNx:H passivation alone has difficulty in accomplishing such a task.